1. Field of the Invention
This invention relates in general to a signal processing apparatus used in a rotary-head, digital tape recorder and, more particularly, to signal processing apparatus used in recording and playing back 4-channel PCM audio signals.
2. Description of the Prior Art
Six different recording modes are employed in the field of rotary-head, digital tape recorders (R-DAT), and the standard mode is the 48k mode adapted to record and play back a 2-channel signal. In the 48k mode, the sampling frequency is 48 kHz, the quantizing bit number is 16 bits for linear quantization, the tape speed is 8.15 mm/s, and the drum speed is 2000 rpm. There is also an optional 32k-LP mode adapted to record and play back 2-channel signals. In the 32k-LP mode, the sampling frequency is 32 kHz, the quantizing bit number is 12 bits for nonlinear quantization, and the drum speed is 1000 rpm. In the 32k-LP mode, only one half of the amount of information can be accommodated compared with the 48k mode, and the data transmission rate is also reduced by one half. Therefore, the tape speed is one half the speed in the 48k mode, and a long recording time is possible.
In the field of R-DAT, a mode for recording and playback of a 4-channel PCM audio signal is established. In the 32k-4CH mode, the sampling frequency is 32 kHz and the quantizing bit number is 12 bits for nonlinear quantization, as in the 32k-LP mode. The information amount, however, because of using four channels, is the same as that of the standard or 48k mode, with the same tape speed and the same drum revolution rate as those described above.
In the field of R-DAT, error correction is carried out in each unit of 8-bit data, typically called a symbol, and this requires conversion of the original 16-bit data or 12-bit data into symbols. In the case of 16-bit data, the upper significant 8 bits and the lower significant 8 bits are formed into two different symbols. In the case of 12-bit data, however, which is not an integral multiple of 8 bits, a plurality of 12-bit data are converted into a plurality of symbols. FIG. 1 represents a conversion rule for converting 12-bit words to symbols in the 32k-LP mode. Eight 12-bit words, L0, R0, L1, R1, L2, R2, L3, R3, for example, of the PCM audio signal are converted into twelve words of 8 bits each, representing twelve symbols. The remaining PCM audio signal consisting of words L4, R4, . . . , L959, R959 are also converted into symbol units in the same manner.
FIG. 2 represents a conversion rule for word-to-symbol conversion in the 32k-4CH mode. Reference symbols A0, B0, C0 and D0 denote PCM audio signals of twelve bits each for four channels, A to D, respectively. It will be appreciated from the apparent differences between FIGS. 1 and 2 that the word-to-symbol conversion rule is different in the 32k-LP mode and the 32k-4CH mode. Furthermore, the 32k-LP mode employs a signal processing period Td of 60 ms, which is equal to the rotation cycle of the drum. In the 32k-4CH mode, a signal processing period of Td/2 of 30 ms is employed.
In addition to the standard 48k mode, two systems are known for establishing the R-DAT 4-channel mode. One is a system wherein the signal processor is designed to be operative with a sampling frequency of 32 Khz and an address generating circuit operative under the aforementioned 4-channel mode conversion rule. The other system advantageously utilizes the fact that the data transmission is identical between the 32k-4CH mode and the standard 48k mode and activates its processor in the 48k mode. Also, the other system scrambles data, at the input side of the processor during recording or at the output side of the processor during playback, to perform the 12-to-16 data conversion and, at the same time, the conversion of the sampling frequency.
The first system uses an address generating circuit or other equivalent circuit peculiar to the 4-channel mode, so that it has the drawback that the scale of the hardware of the processor is increased. The second system, which requires some additional circuits for the 4-channel mode in the processor, makes it necessary to modify the existing processor that is typically in the form of an LSI, thereby requiring redesign of the LSI.
In addition, if multi-channel digital audio signals other than a stereo system can be recorded or reproduced with the digital audio tape recorder of this kind, as is done using a conventional audio tape recorder configured to record and reproduce an analog signal, the usefulness of the digital audio tape recorder will be improved significantly.
Moreover, if the digital audio tape recorder is capable of recording and reproducing a digital audio signal between channels, then mixing processing, edit processing, and the like of the digital audio signal may be carried out as desired, and the usefulness of the recorder will be further improved.